Resistance furnace



Oct. 19, 1965 R. J. DIEFENDORF RESISTANCE FURNACE Filed June 4, 1963 /nvemor Passe J. Diefendorf by W K His Aflarn ey- United States Patent 3,213,177 RESISTANCE FURNAQE Russell .1. Diefendorf, liallston Spa, N.Y., assignor to General Electric Company, a corporation of New York Filled .Iune 4, 1963, Ser. No. 285,298 9 Claims. (El. 13-31) This invention relates to resistance furnaces and more particularly to elevated temperature resistance furnaces which employ a heating element or tube of graphite.

Present resistance furnaces, which employ a heating element of graphite for operation at elevated temperatures, employ a helical strip or tube. The strip is subject to creep at elevated temperatures while the graphite tube has a relatively short hot zone with the remainder of the tube functioning as electrodes from the leads to supply the necessary power to the furnace. Thus, it would be desirable to provide a compact furnace with a sharp temperature gradient providing rapid heat up. The present invention is directed to such a compact furnace employing an elevated temperature heating element and a pair of electrodes whose combined thicknesses are substantially less than the length of the hot zone of the furnace.

It is an object of my invention to provide a compact resistance furnace employing a heating element of graph ite.

It is another object of my invention to provide a compact resistance furnace employing a high temperature heating element with a pair of pyrolytic graphite electrodes.

It is a further object of my invention to provide a compact resistance furnace employing a high temperature heating element with a carbonaceous electrode assembly.

In carrying out my invention in one form, a resistance furnace comprises a high temperature heating element, a pair of pyrolytic graphite electrodes in electrical con tact with opposite ends of said element, said heating element being longer than the combined thicknesses of said pair of pyrolytic graphite electrodes, thermal insulation surrounding said element, a casing surrounding said thermal insulation, a metal electrode in contact with each end of said casing and electrically insulated therefrom, an electrical lead connected to each of said metal electrodes, each of said pyrolytic graphite electrodes in elec trical contact with one of said metal electrodes, and an apertured cover at each end of said casing.

These and various other objects, features and advantages of the invention will be better understood from the following description taken in connection with the accompanying drawings in which:

FIGURE 1 is a sectional view through a furnace embodying my invention; and

FIGURE 2 is a sectional view through a portion of a modified furnace.

In FIGURE 1 of the drawing, a furnace 1t embodying my invention is shown which comprises a central heating element 11 of high temperature material in the form of a hollow tubular configuration of commercial graphite. Other solid or hollow configurations of high temperature material may also be employed for the heating element. At each end of heating element or tube 11 is positioned a pyrolytic graphite electrode 12 in mechanical and electrical contact therewith. Each of these electrodes 12 is shown in the form of a ring with a flange fitting against the end of tube 11. However, any suitable electrode configuration can be employed. A graphite electrode 13 which has a larger length-to-area ratio for the electric current path is positioned adjacent electrode 12 and in electrical contact therewith. In this figure of the drawing, pyrolytic graphite electrode 12 and 3,213,177 Patented Get. 19, 1965 graphite electrode 13 form an electrode assembly for constant heating of graphite tube 11 during the operation of furnace 1t).

Tube 11 is insulated from heat loss by suitable insulation 14 in the form of a blanket of graphite felt or thermal black. In the particular embodiment of FIGURE 1 of the drawing, furnace 10 is adapted to be employed as a vacuum furnace. An outer metallic casing 15, for example, of brass, is positioned around insulation 14. Casing 15 is suitably cooled by water coils 16 surrounding the casing. At opposite ends of the casing 15, there is provided end cover structures 17 and 18 which each include a water-cooled electrode 19 in electrical contact with electrode 13. Electrode 19 is threaded at its inner periphery 20. An inner ring member 21 is threaded to threads 20 of electrode 19. A plurality of bolts 22 are inserted through the openings in cover 17 and threaded into openings in upper flange 23 of casing 15 to mechanically position upper graphite electrode 13 against electrode 12. Upper electrode 13 is also provided with a plurality of openings 24 to provide passageways to insulation 14. A plate 25 is aflixed as by screws 26 to the upper surface of member 20. An 0 ring 27 is provided near the outer periphery of plate 25 to produce an effective seal. A viewing window 28 is shown positioned centrally in plate 25.

An electrical lead 29 is shown in electrical contact with water-cooled electrode 19. The lead is connected to one terminal of a power source (not shown). Cover 17 is secured to the upper end of casing 15 by means of a plurality of bolts 22 which are inserted through a plurality of openings in cover 17 and which are threaded in threaded openings in a flange 23 on the upper end of casing 15. Electrically and thermally insulating material 30 is provided between cover 17 and flange 23, and within and surrounding openings in cover 17.

Lower cover 18 has a water-cooled electrode 19 near its outer periphery. Cover 18 has a central plate portion 31 with an aperture 32 therein centrally located into which a gas inlet line 33 is afilxed. A plurality of bolts 22 are inserted through openings in lower cover 18 and are threaded in threaded openings in lower flange 23 of casing 15 to secure lower cover 18 to casing 15. Insulation 30 can also be provided between cover 18 and flange 23 and within and surrounding the openings in cover 18. A second lead 34 is connected to water-cooled electrode 19 in cover 18 and to the other terminal of the power source (not shown) to complete the electrical circuit to the furnace. The inner periphery of the water-cooled electrode 19 is tapered inwardly towards casing 15 to provide support for graphite electrode 13 which is tapered in similar fashion and fits thereagainst.

Gas inlet line 33 is connected to a suitable source of gas, for example, a hydrocarbon gas such as methane, which it is desired to introduce into tube 11 of furnace 16. A graphite tube 35 is shown positioned within tube 11 for the purpose of depositing a pyrolytic graphite coating therein. With such a tube 35 in position, the gas flows from inlet line 33 into the interior of tube 35. At the upper end of furnace 11) an opening 36 is provided, for example, through plate 25 of cover 17. A tube 37 is positioned in opening 36 and connected to a pump 38 to provide for evacuation of furnace 10 to a desired subatmospheric condition during operation.

In FIGURE 2 of the drawing, a portion of a modified furnace structure is shown wherein a pyrolytic graphite electrode 39 in the form of a ring is positioned adjacent and in electrical contact with each end of tube 11. The tapered, water-cooled electrode 19 is positioned adjacent and in electrical contact with pyrolytic graphite electrode 39. In this embodiment, a pair of pyrolytic graphite electrodes, whose combined thicknesses are less than the length of the hot zone, are employed rather than the electrode assembly in FIGURE 1.

I found unexpectedly that in order to produce a compact furnace which has a constant temperature in its heating zone, a pyrolytic graphite electrode must be in contact with each of the opposite ends of the heating zone tube. While graphite or pyrolytic graphite pnovides a suitable heating element, the preferred heating element is graphite infiltrated with pyrolytic graphite. Thus, the two separate problems of compactness and uniform heating were solved by such a structure. Generally, previous electrodes are of substantial length in comparison to the length of the hot zone in a heating furnace. It is not unusual to find a furnace arrangement wherein both electrodes occupy a substantial portion of the entire length of the furnace. Pyrolytic graphite electrodes solved this problem wherein the electrodes occupy a very small portion of the length of the furnace and the heating zone is substantial in length in relation to the electrodes.

These pyrolytic graphite electrodes provide a number of advantages. First, they provide a small length-overarea relationship for the electric current path in the electrodes. In this manner, the electrodes occupy a small portion of the length of the furnace in relation to the length of the hot zone. I found that these pyrolytic graphite electrodes raise the temperature of the hot zone rapidly to the desired temperature and maintain a constant temperature along the hot zone. Furthermore, I found that if the current flow is in the c direction of the pyrolytic graphite electrodes, that is the c direction is perpendicular to the longitudinal axis of the heating element, a good compact furnace with desirable characteristics was produced. Furthermore, I found that if I employ pyrolytic graphite electrodes with the current flow in the a direction, that is the a direction is perpendicular to the longitudinal axis of the heating element, an even more efficient furnace with less wattage loss was produced. Additionally, the desirable characteristics of compactness and uniform heating are increased by doping the pyrolytic graphite electrodes with small weight percentages of boron. For example, 0.5 to 1.0 atomic weight percent of boron is suitable.

The above-desirable characteristics for the furnace are produced by employing a pyrolytic graphite electrode having a small length-over-area relationship for the current path and a commercial graphite electrode having a larger length-over-area relationship for the electric current path adjacent to and in electrical contact with the pyrolytic graphite electrodes. These electrodes form an electrode assembly which is in contact with the heating zone or tube and the water-cooled metallic electrodes. The additional advantage of such a structure is during the application of higher temperatures such as, for example, 2000 C. to 2500 C. in the tube, the pyrolytic graphite is adjacent to the tube and in contact with the higher temperature range .whereas the commercial graphite electrode is maintained at a lower temperature which is suitable from a power loss standpoint to this particular type of material. In this manner, the temperature drop from the high temperature within the tube and adjacent the pyrolytic graphite electrode across to the water-cooled electrode is more gradual by employing the commercial graphite electrode in this electrode structure.

The modified furnace construction wherein pyrolytic graphite electrodes are in electrical contact with the tube and the water-cooled electrodes provides the same advantages for the furnace as are obtained from the electrode assembly. However, the pyrolytic graphite electrodes are more desirable when the complete temperature drop is required in a short distance.

I found that I can determine the dimensions of electrode materals to minimize heat loss out of these electrodes. A formula for such determinations, which is also Q. applicable to pyrolytic graphite electrodes, and pyrolytic graphite and graphite electrode assemblies is as follows:

1) THT. ]1

In the above formula, L is the length of the electrode, A is the area of the electrode, T is the hot zone temperature of the electrode, T is the temperature of the electrode at the water-cooled electrode, K is the thermal conductivity, 9 is the resistance in ohms, and I is the current. When both the pyrolytic graphite electrode and the commercial graphite electrode are employed together as an electrode structure, this formula can be used for each of the separate electrodes which make up the electrode assembly. Furthermore, I found that the wattage loss to each water-cooled electrode which is necessary to generate a constant heat zone within the furnace through tube 11, depends upon the following formula:

In the above formula, W is the power loss in watts, T is the temperature of the hot zone of tube 11, T is the temperture of the cold zone or the temperature at the watercooled electrode, b is a geometrical factor of 2 for a constant cross section, 9 is the resistance of the electrode material, K is the thermal conductivity of the electrode material, and I is the current. I found additionally that the hot zone tube should be a thin tube to keep the radial temperature gradient in the tube small and to minimize heat loss to the water-cooled electrodes. In accordance with Formula 2, the watts lost to the water-cooled electrodes is minimized by a minimum value of I. The minimum value of current for a given power generation, the power requirement which is equal to the power lost to the outer casing, in the heating tube is attained by making the resistance as high as possible. The value of the resistance is reflected by using close to the limiting current density of graphite at the highest operating temperature. In this manner, in accordance with the above formulas, the power which is necessary for the furnace to generate a constant temperature through the hot zone, is readily calculated for the furnace construction.

In the operation of furnace 10 shown in FIGURE 1 of the drawing, a thin tube 11 of graphite infiltrated with pyrolytic graphite is employed. A pair of pyrolytic graphite electrodes, whose dimensions are determined in accordance with above Formula 1, are applied at opposite ends of tube 11. Commercial graphite electrodes 13, whose dimensions are also determined in accordance with Formula 1, are positioned in contact with electrodes 12 to form electrode assemblies. A blanket of insulation such as carbon felt I4 is wrapped around the tube 11. This structure is fitted into casing 15, for example, through an open upper end of the casing with lower cover 18 already attached. Subsequently, bolts 22 are inserted through the openings in cover 17 and threaded into the openings in flange 23 of casing 115. Vacuum pump 38 is then employed to reduce the pressure within casing 15 to a desired level, for example, from 0.01 millimeter of mercury to slightly below atmospheric pressure. A hydrocarbon gas, such as methane, is admitted through inlet line 33 into the interior of tube 35 which is positioned Within tube 11. In accordance with Formula 2 above, sufiicient power is generated to heat tube 11 rapidly to the desired constant temperature, for example, to a specific temperature in a range between 2000 C. to 2500 C. for decomposing the gas and depositing the vapor therefrom on the interior of member 35 as pyrolytic graphite. Tube 35 may be omitted and other neutral or reducing gases employed. In the latter event, temperatures of 1000 C. to 2500 C. will normally be selected. Water is circulated through watercooled electrodes 1% and coil 16 to provide suitable cooling for furnace I0.

In the above operation, after a particular thickness of pyrolytic graphite has been deposited on the interior surface of member 35 by thermal decomposition of the hydrocarbon gases, the power is terminated and the apparatus is allowed to cool to room temperature. The apparatus is then returned to atmospheric pressure after which cover 17 is unbolted from casing 15. The apparatus is disassembled to remove coated tube 35 from tube 11 in furnace 10. In this manner, pyrolytic graphite coated composite articles or free-standing bodies can be produced in this furnace. The furnace is also operable as a vacuum furnace or as a resistance furnace at or above atmospheric pressure.

The operation of the furnace in FIGURE 1 of the drawing as modified in accordance with the structure of FIGURE 2, operates in the identical manner with the above-described operation for the apparatus of FIG- URE 1.

While other modifications of this invention and variations thereof may be employed within the scope of the invention have not been described, the invention is intended to include such that may be embraced within the following claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A resistance furnace comprising a high temperature heating element of graphite infiltrated with pyrolytic graphite, a pair of pyrolytic graphite electrodes in electrical contact with opposite ends of said element, said electrodes having their a direction perpendicular to the longitudinal axis of said element, said heating element being longer than the combined thicknesses of said pair of pyrolytic graphite electrodes, thermal insulation surroundin" said element, a casing surrounding said thermal insulation, a metal electrode in contact with each end of said casing and electrically insulated therefrom, an electrical lead connected to each of said metal electrodes, each of said pyrolytic graphite electrodes in electrical contact with one of said metal electrodes, and an apertured cover at each end of said casing.

2. A resistance furnace comprising a high temperature heating element of graphite infiltrated with pyrolytic graphite, a pair of pyrolytic graphite electrodes in electrical contact with opposite ends of said element, said electrodes having their a direction perpendicular to the longitudinal axis of said element, a graphite electrode in electrical contact with each of said pyrolytic graphite electrodes, said heating element being longer than the combined thicknesses of said pairs of electrodes, thermal insulation surrounding said element, a casing surrounding said thermal insulation, a metal electrode in contact with each end of said casing and electrically insulated therefrom, an electrical lead connected to each of said metal electrodes, each of said graphite electrodes in electrical contact with one of said metal electrodes, and an apertured cover at each end of said casing.

3. A resistance furnace comprising a high temperature heating element of graphite infiltrated with pyrolytic graphite, a pair of boron doped pyrolytic graphite electrodes in electrical contact with opposite ends of said element, said electrodes having their a direction perpendicular to the longitudinal axis of said element, said heating element being longer than the combined thicknesses of said pair of boron doped pyrolytic graphite electrodes, thermal insulation surrounding said element, a casing surrounding said thermal insulation, a metal electrode in contact with each end of said casing and electrically insulated therefrom, an electrical lead connected to each of said metal electrodes, each of said boron doped pyrolytic graphite electrodes in electrical contact with one of said metal electrodes, and an appertured cover at each end of said casing.

4. A resistance furnace comprising a high temperature heating element of graphite infiltrated with pyrolytic graphite, a pair of boron doped pyrolytic graphite electrodes in electrical contact with opposite ends of said element, said electrodes having their a direction perpendicular to the longitudinal axis of said element, a graphite electrode in electrical contact with each of said pyrolytic graphite electrodes, said heating element being longer than the combined thicknesses of said pairs of electrodes, thermal insulation surrounding said element, a casing surrounding said thermal insulation, a metal electrode in contact with each end of said casing and electrically insulated therefrom, an electrical lead connected to each of said metal electrodes, each of said pyrolytic graphite electrodes in electrical contact with one of said metal electrodes, and an apertured cover at each end of said casing.

5. A resistance furnace comprising a high temperature heating element, a pair of pyrolytic graphite electrodes in electrical contact with opposite ends of said element, said heating element being longer than the combined thicknesses of said pair of pyrolytic graphite electrodes, thermal insulation surrounding said element, a casing surrounding said thermal insulation, a metal electrode in contact with each end of said casing and electrically insulated therefrom, an electrical lead connected to each of said metal electrodes, each of said pyrolytic graphite electrodes in electrical contact with one of said metal electrodes, and an apertured cover at each end of said casing.

6. A resistance furnace comprising a high temperature heating element, a pair of pyrolytic graphite electrodes in electrical contact with opposite ends of said element, a graphite electrode in electrical contact with each of said pyrolytic graphite electrodes, said heating element being longer than the combined thicknesses of said pairs of electrodes, thermal insulation surrounding said element, a casing surrounding said thermal insulation, a metal electrode in contact with each end of said casing and electrically insulated therefrom, an electrical lead connected to each of said metal electrodes, each of said graphite electrodes in electrical contact with one of said metal electrodes, and an apertured cover at each end of said casing.

7. A resistance furnace comprising a high temperature heating element of graphite, a pair of pyrolytic graphite electrodes in electrical contact with opposite ends of said element, said heating element being longer than the combined thicknesses of said pair of pyrolytic graphite electrodes, thermal insulation surrounding said element, a casing surrounding said thermal insulation, a metal electrode in contact with each end of said casing and electrically insulated therefrom, an electrical lead connected to each of said metal electrodes, each of said pyrolytic graphite electrodes in electrical contact with one of said metal electrodes, and an apertured cover at each end of said casing.

8. A resistance furnace comprising a high temperature heating element of pyrolytic graphite, a pair of pyrolytic graphite electrodes in electrical contact with opposite ends of said element, said heating element being longer than the combined thicknesses of said pair of pyrolytic graphite electrodes, thermal insulation surrounding said element, a casing surrounding said thermal insulation, a metal electrode in contact with each end of said casing and electrically insulated therefrom, an electrical lead connected to each of said metal electrodes, each of said pyrolytic graphite electrodes in electrical contact with one of said metal electrodes, and an apertured cover at each end of said casing.

9. A resistance furnace comprising a high temperature heating element of graphite infiltrated with pyrolytic graphite, a pair of pyrolytic graphite electrodes in electrical contact with opposite ends of said element, said heating element being longer than the combined thicknesses of said pair of pyrolytic graphite electrodes, thermal insulation surrounding said element, a casing surrounding said thermal insulation, a metal electrode in contact with each end of said casing and electrically insulated therefrom, an electrical lead connected to each of said metal electrodes, each of said pyrolytic graphite electrodes in electrical contact with one of said metal electrodes, and an apertured cover at each end of said casing.

References Cited by the Examiner Williams M 2l9-422 Ridgway 13-20 Grisdale et al 117--226 X Kistler 1320 Buck et al 219427 Gentner 11'7-46 X Planer et a1. 117--226 X Drewett 117226 X Mulcihy 1322 X Diefendorf 23209.1

Kraus 60-35.6

RICHARD M. WOOD, Primary Examiner. 

5. A RESISTANCE FURNACE COMPRISING A HIGH TEMPERATURE HEATING ELEMENT, A PAIR OF PYROLYTIC GRAPHITE ELECTRODES IN ELECTRICAL CONTACT WITH OPPOSITE ENDS OF SAID ELEMENT, SAID HEATING ELEMENT BEING LONGER THAN THE COMBINED THICKNESSES OF SAID PAIR O PYROLYTIC GRAPHITE ELECTRODES,THERMAL INSULATION SURROUNDING SAID ELEMENT, A CASING SURROUNDING SAID THERMAL INSULATION, A METAL ELECTRODE IN CONTACT WITH EACH END OF SAID CASING AND ELECTRICALLY INSULATED THEREFROM, AN ELECTRICAL LEAD CONNECTED TO EACH OF SAID METAL ELECTRODES, EACH OF SAID PYROLYTIC GRAPHITE ELECTRODES IN ELECTRICAL CONTACT WITH ONE OF SAID METAL ELECTRODES, AND AN APERTURED COVER AT EACH END OF SAID CASING. 